This invention relates to polymer coatings for a substrate. In one aspect, the invention relates to precoating a substrate with a material which improves the adhesion of the polymer coating.
Many polymer coatings are known. Polyolefin coatings, for example, polyethylene, polypropylene or copolymers thereof with other unsaturated monomers or with acrylates such as acrylic acid or methacrylic acid, or terpolymers like EPDM, containing xe2x80x94Cxe2x95x90O, xe2x80x94COOR, etc., or oxidized polyolefins, can be used to provide very high quality coating materials which will protect structures from corrosion. Mixtures of materials, for example polyolefin and epoxy, polyolefin and polyester, or polyolefin and engineering plastics for example, PEEK, or PEAK can also be used. If desired, engineering plastics such as PEEK or PEAK can be used alone.
The major problem encountered with the use of any of these materials is adhesion to the substrate. For coatings, substrates can be classified as metallic and nonmetallic. The most common metallic substrate is steel. Common nonmetallic substrates are concrete, or surfaces which have been previously coated with vinyl, epoxy, or common paints.
In one prior art coating technique, the substrate is heated to 400 F. and then dipped into the powdered polyolefin. In another prior art technique, powdered polyolefin is flocked on the surface of the substrate, generally by electrostatic means, and then cured at 400 F. or above for half an hour or more. This method can only be practiced in a shop or lab and is not suitable for large structures such as bridges, railcars or other large structures which cannot be dismantled or transported to the shop. Also, the technique is not suitable for practice with all substrates, because many nonmetallic substrates, for example vinyl or epoxy coated surfaces may char at that temperature.
Previously, the use of thermoplastics for the purpose of field applied corrosion protection and/or other functional performance has been difficult and provided inconsistent results. The required substrate temperature was very high, difficult to achieve in the field condition. This often resulted in inconsistent adhesion, delamination and premature failure of the topcoat thermoplastics. To further complicate the use of thermoplastics, adhesion to the surface was also dependent upon demanding preparation requirements such as concentrated sandblasting in order to provide a sufficient mechanical profile. The bonding between the substrate and the topcoat was physical in nature.
Given the difficulty of preheating large structures to elevated temperature, and the expenses and other inherent difficulties associated with extensive surface preparation, the introduction and application of a flame-activated primer that eliminates preheating and greatly minimizes the surface preparation would be of great benefit.
To facilitate coating large structures, flame spray application was developed for use in the field. There are several different types, for example, High Velocity Oxy-Fuel (HVOF), Propane torch, Arc Spray, and Vacuum Plasma Spray, are available in the market. However, the simplest one is powder flame spray.
Powder flame spray is a process that deposits finely powdered materials onto a surface in a molten or semi-molten state. The powdered material is fed at a controlled rate to the flame spray gun, which heats the material to the molten or semi-molten state with combustion gases or electricity. Compressed gas then propels the particles to the surface at a speed generally in the range of 10 to 20 meters per second. The particles hitting the surface flatten and conform to the surface, forming the coating. Generally, separate heating or curing is not required.
The resulting coatings are used on a wide variety of applications, from aircraft engine components and biomedical prostheses to bridges and pumps. The coating has several advantages; it covers sharp edges and welds well; it can be used as soon as it cools; there are no runs, drips, sags, or incomplete cure. The coating can also be quench cooled, and small repairs can be done by heating the part to reflow the polymer. The coatings, especially when containing a polyolefin component, have high chemical, impact, and abrasion resistance. Also, the coatings are applied in a solvent-free procedure, avoiding VOC emissions.
Despite these advantages and lucrative applications, flame spray technology has not flourished because it also has several disadvantages.
In order to provide reasonably good adhesion, the substrate must be activated by preheating to 400 F. or above before flame spraying the powder. This is difficult (and expensive) to do with large structures such as bridges. It is also generally necessary to clean rusted steel substrate to at least Standard SP-10 and to remove any old coating, such as vinyl, epoxy or other paint as it cannot withstand such preheating temperature. These cleaning steps can add substantial expense and time requirements. Post application problems include damage to the polymer coating caused by excessive heat exposure on the preheated substrate, poor adhesion caused by inadequate preheating, shrinkage, poor dimensional stability, poor or no adhesion to the substrate.
It is an object of this invention to provide a coating method and system that avoids the need to preheat the substrate.
It is another object of this invention to provide a coating method and system that avoids the need to carefully pre-cleaning the substrate.
It is a further object of this invention to provide a coating method and system in which the coating has good dimensional stability and exhibits little or no shrinkage on cooling.
It is yet another object of this invention to provide a highly protective and long-lived coating for a substrate.
In one embodiment, the invention comprises a composition based on an epoxy resin and amine hardener that may be used to prime a substrate to facilitate subsequent bonding of a polyolefin material which may be sprayed thereon. The invention is especially useful where open flame is used, i.e., where the polyolefin is flame sprayed on the primed surface. A monolayer thickness of the primer is preferred. However, the primer can be air sprayed as a thin coat of visual thickness on the substrate and will provide very good adhesion of the topcoat with minimal surface preparation of the substrate.
The primer composition comprises an epoxy resin, preferably of low molecular weight and/or low viscosity and an amine hardener, part of which becomes highly reactive on exposure to open flame, thus providing near instant cure of the composition under flame spray conditions. Optionally, the primer composition can include a solvent to facilitate spray where required.
The invention is unique that it provides strong adhesion to steel, even where the surface preparation is less than ideal, as well as to concrete and other substrates. It also has strong adhesion to the topcoat polyolefinic material, especially to a functionalized one. The composition withstands open flame and does not char under polyolefin flame spray conditions.
The physical advantages provided by the coating system of present invention are: (1) the adhesion of the topcoat to the substrate, (2) the adhesion of the primer to polyolefin topcoat, (3) the dimensional stability of the topcoat, (4) an exceptional peel strength (both 180 and 90 degree peel) of the topcoat, (5) an exceptional pull strength (hate test) of the topcoat, (6) the provision for very good cathodic protection of the substrate, and (7) the prevention of water permeation underneath the coating system.
The advantages provided by the application method of the invention are:: (1) instant cure on exposure to open flame, and concurrent bonding with the substrate and topcoat polyolefins, (2) avoiding the preheating of the substrate, (3) avoiding careful cleaning of the substrate, (4) the ability to apply the coating system on a rusty substrate, (5) ease in avoiding runs and sags in the coating as it bonds on cure, and (6) the long working pot-life of the primer.